In many industries, like the petrochemical and chemical industries, contact of reaction fluids with a catalyst in a reactor under suitable temperature and pressure conditions effects a reaction between the components of one or more reactants in the fluids. Most of these reactions generate or absorb heat to various extents and are, therefore, exothermic or endothermic. The heating or chilling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include among other things: poor product production, deactivation of the catalyst, production of unwanted byproducts and, in extreme cases, damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone.
Many arrangements seek to overcome the negative effects of endothermic chilling by supplying heat to the reaction or of exothermic heating by removing heat from the reaction. More traditional methods employ multiple stages of heating between adiabatic reaction stages. Other methods use in-situ heating via simultaneous reactions or indirect heat exchange to maintain an isothermal or other temperature profile within the reaction zone. U.S. Pat. No. 5,525,311 provides an example of indirect heat exchange with a heat exchange fluid to control the temperature profile within a reaction zone.
A variety of processes can employ indirect heat exchange with a reaction zone to control temperature profiles within the reaction zone. Common examples of hydrocarbon conversion reactions include: the aromatization of hydrocarbons, the reforming of hydrocarbons, the dehydrogenation of hydrocarbons, the oxidation of hydrocarbons and the alkylation of hydrocarbons.
Low Prandtl number heat transfer fluids have long provided benefits for improving temperature control in reactions. Low Prandtl number heat transfer fluids are used in several industries to provide cooling for shell and tube heat exchanger arrangements. Suitable types of low Prandtl number fluids include alkali liquid metals such as sodium, lithium, and potassium and include molten salts such as nitrates and carbonates. These heat transfer fluids combine high heat capacity with high thermal conductivity. British patent 2170898 generally discloses the use of sodium as a heat transfer medium in high temperature reactions including heat recovery from furnace installations, high pressure nuclear reactors, coal gasification, coal conversion, and water disassociation. U.S. Pat. No. 4,549,032 discloses the use of molten salt as an indirect heat transfer medium with a dehydration of styrene. German patent DE 2028297 discloses the use of an alkaline metal as a heat transfer medium in a process for producing alkenes and aromatics by cracking aliphatic hydrocarbons. The liquid metals are specifically used due to their high heat transfer capacity that permits utilization of small heating surfaces.
One problem with the use of most low Prandtl number heat transfer fluids is the need to maintain absolute segregation between the heat transfer fluid and the reactants. The molten salts and liquid metals, used to provide most low Prandtl number heat transfer fluids, typically act as catalyst poisons. Leakage of the high heat capacity stream across the narrow reaction channels can result in premature or immediate catalyst deactivation. Depending upon its composition, such heat transfer fluids may immediately deactivate and/or permanently kill the catalyst. Minute concentrations of the various molten salts or liquid metals can bring about such catastrophic catalyst problems. In many cases even the most minor amounts of leakage of such heat transfer fluids from the heat exchange channels to the reaction channels can quickly shut down a process. Therefore, successful processes must prevent any leakage from the heat exchange channels to the reaction channels when using most catalysts since they are likely to have a high sensitivity to low Prandtl number heat transfer fluids.
It is known to accomplish indirect heat exchange for processes with thin plates that define reaction and heat exchange channels. The channels alternately retain catalyst and reactants in one set of channels and a heat transfer fluid in adjacent channels for indirectly heating or cooling the reactants and catalysts. Heat exchange plates in these indirect heat exchange reactors can be flat or curved and may have surface variations such as corrugations to increase heat transfer between the heat transfer fluids and the reactants and catalysts. Many hydrocarbon conversion processes will operate more advantageously by maintaining a temperature profile that differs from that created by the heat of reaction. In many reactions, the most beneficial temperature profile will be obtained by maintaining substantially isothermal conditions. In some cases, a temperature profile directionally opposite to the temperature changes associated with the heat of reaction will provide the most beneficial conditions. For such reasons it is generally known to contact reactants with a heat exchange medium in cross flow, cocurrent flow, or countercurrent flow arrangements. A specific arrangement for heat transfer and reactant channels that offers more complete temperature control can again be found in U.S. Pat. No. 5,525,311; the contents of which are hereby incorporated by reference. Other useful plate arrangements for indirect heat transfer are disclosed in U.S. Pat. No. 5,130,106 and U.S. Pat. No. 5,405,586.
Most plate and channel arrangements have substantial limitations on the amount of pressure differential that the plates can withstand. The most useful plates for such arrangements are formed from thin sheet which facilitates a fabrication of plates in the most useful arrangements for heat transfer. Useful plate configurations will usually contain corrugations for the defining heat transfer and reaction channels. Normally the thickness of the plates does not exceed 2 millimeters. This thickness constraint limits the differential pressures that the plates can withstand. Accordingly, the pressure of the heat transfer fluid is often dictated by pressure requirements for the process since too great of a difference will cause distortion of the plates which can lead to collapse of the channels and leakage between the heat transfer channels and the reaction channels.
The inherent problems of withstanding a high pressure differential across channels of a plate and channel type heat exchanger arrangement has made it difficult to use such arrangements for high temperature chemical reactions unless the reaction process also operates at high pressure. When considering such applications, traditional heat transfer fluids that would be used for such processes--such as methane, hydrogen, and steam--have the necessary thermal stability, but they are not practical from an efficiency standpoint. This is particularly troublesome when carrying out reactions that are highly sensitive to operating temperatures. For such reactions the difference between the temperature of the heating or cooling fluid must be close to the desired temperature for maximizing the conditions for the reaction. This approach temperature is particularly difficult to maintain without circulating large amounts of most traditional heat transfer fluids. As long as the process requires low pressure, the volumetric heat capacity of such fluids increases the energy requirements for circulation of the heat transfer medium to excessive levels. However, raising the pressure of such heat transfer fluids is not practical since it may compromise the integrity of the plate and channel structure in the manner previously described.
It is therefore, an object of this invention to improve the efficiency of heating reactants in processes and heat transfer arrangements that use thin plate arrangements.
It is another object of this invention to extend the practicality of using of plate and channel heat transfer arrangements to high temperature chemical reactions.